On Saturday and Sunday morning, employees and travelers at Houston's Bush Intercontinental Airport were treated to a horrifying sight: hundreds of birds dropping out of the sky, having seizures, and then dying slowly and painfully.

Houston TV station KHOU investigated the matter and found that United Airlines, in cooperation with the airport, hired a pest control contractor to "reduce the health and safety risk" of the birds using a poison called Avitrol. The airline placed poisoned bait trays throughout their hangar full of corn tainted with the substance.

The result, the TV station reports, was hundreds of grackles and pigeons falling to the ground and having seizures that lasted as much as an hour before they finally died.

"These deaths look anything but humane," said Dr. John Hadidian, Senior Scientist with the Humane Society of the United States.

One of the effected birds, a great-tailed grackle, took a full hour to die—sometimes struggling to move its legs, sometimes appearing paralyzed with its beak open for several minutes at a time.

"The birds that are dying after ingesting this compound are suffering and in great distress," Dr. Hadidian said.

It's no secret that bird strikes can pose a serious risk to aircraft. That plane that landed in the Hudson River back in 2009 was forced down because it was struck by a bird.

But critics say that the use of Avitrol — which is banned entirely in several states — was not a humane way to deal with the bird problem at the airport, and the resulting display of birds collapsing, seizing and dying painfully in front of scores of people was certainly a macabre one.

USA Today reports that the makers of Avitrol say the birds were not in pain, and those who ingest a small amount "recover with no lasting effects." Clearly, that's not what happened here. United says it hires pest control companies to handle bird abatement there about once a year. Other bird control methods used by airports include loud noises and even birth control pills in food.

Go to KHOU's website to see video of the bird deaths at the airport, but be warned that it's not one of the more pleasant things you'll see all day.